The invention relates to an energy filter device for beams used in low beam therapy for controlling the beam intensity and the dose strength of the beam.
Ion beams, that is beams of protons and heavier ions, are used for beam therapy for tumor tissues. Beams of this type have the advantage over photons that they have a considerably better depth dose profile. While, in the case of photon radiation, the dose decreases as the penetration depth increases, it rises slowly in the case of ions and falls away steeply after a sharp maximum, the so-called Bragg peak. The position of this maximum must be distributed accurately over the target volume, in order to concentrate the dose on the tumor and at the same time to reduce the integral dose in the healthy tissue. This also applies to the laser-induced ion beams. Recently, it has been shown that high-intensity, pulsed laser beams can be used to produce relatively tightly focused proton beams from films. However, it has been found that the laser-induced radiation has an energy spectrum which is very poor for cancer therapy, and thus a poor depth dose profile.
The energy spectrum can be improved by masking out a narrow energy band, and destroying the rest of the spectrum, by means of a magnet spectrometer. However, in the case of this selection method, the vast majority of the protons that are produced, specifically more than 95% of them, are filtered out and destroyed without being used.